On my 98 v70 I had the intermittent stumbling with some rpm drop, no codes, but bad gas mileage. I replaced the thermostat and coolant temp sensor, but used a CTS that is not original volvo dealership, now I have the CTS code and the stumbling problem again, it ran great for the first 50 miles. Does it really matter which brand I use for this, its only a brass temp sensor. Im leaning toward that being the problem since the car usually hates other aftermarket parts( spark plugs, rotors, pads etc..).